HookThe poison London's detectors couldn't see
On the afternoon of 1 November 2006, former Russian security officer Alexander Litvinenko drank most of a pot of green tea in the Pine Bar of the Millennium Hotel, Mayfair. Within days he was in University College Hospital with textbook radiation sickness — hair gone, bone marrow failing — while test after test came back clean. Hospital detectors hunt gamma radiation, and there was none to find. Only hours before he died on 23 November did a sample reach the Atomic Weapons Establishment, whose scientists identified polonium-210: a nuclide that emits alpha particles and almost nothing else. Alpha radiation is stopped by a sheet of paper, a few centimetres of air, the dead outer layer of your own skin — or the wall of a teapot. The murder weapon was invisible to every standard detector it passed, including the airport monitors it was smuggled through.
Inside a body, that shielding logic flips into reverse. Trapped among living cells, each decay dumps its entire energy at point-blank range, and the public inquiry heard that Litvinenko had swallowed billions of becquerels' worth — billions of nuclei disintegrating every second. Scotland Yard reconstructed the crime by following the contamination: polonium traces on the teapot, in hotel rooms, on restaurant tables and aircraft seats, a radioactive trail across London that ordinary detective work could never have seen. The 2016 inquiry under Sir Robert Owen mapped the murder decay by decay. P4 is the physics that makes every line of that story make sense: what an unstable nucleus is, why its decay is random yet statistically exact, what alpha, beta and gamma actually are, how a balanced nuclear equation tracks them, what a half-life of 138 days means — and why contamination and irradiation are entirely different hazards.
ModelThe atom by the numbers
An atom is about \(1 \times 10^{-10}\) m across — ten billion of them to the metre. Nearly all of that is empty space: the nucleus, which holds the protons and neutrons, has a radius less than 1/10,000 of the atom's. Scale an atom up to a football stadium and the nucleus is a marble on the centre spot. Relative charges: proton +1, neutron 0, electron −1. Protons and neutrons have roughly equal mass; the electron is nearly two thousand times lighter, which is why almost all of an atom's mass sits in the nucleus. An atom carries equal numbers of protons and electrons, so it is neutral overall; strip away an outer electron and you have made a positive ion.
The electrons are arranged at different distances from the nucleus — energy levels. Those arrangements can change: an electron that absorbs electromagnetic radiation moves to a higher level, further from the nucleus; when it falls back it emits radiation. What never changes without changing the element itself is the number of protons: proton number is the atom's identity card.
ModelIsotopes and nuclear notation
Nuclear notation compresses the whole census into two numbers around a symbol. The atomic number Z, written at the bottom, counts protons; the mass number A, at the top, counts protons plus neutrons — so the neutron count is A − Z. Isotopes are atoms of the same element, with the same proton number, but different numbers of neutrons and therefore different mass numbers. Chlorine-35 and chlorine-37 are chemically identical twins of different mass.
What isotopes are not is equally stable. Carbon-12 will sit unchanged forever; carbon-14's nucleus is unstable. That is the entire origin of radioactivity: some proton–neutron mixtures hold together indefinitely, others do not, and an unstable nucleus sooner or later rebalances itself by throwing something out.
Read \({}^{210}_{84}\mathrm{Po}\) like a database record. Z = 84: 84 protons and — since the atom is neutral — 84 electrons. A = 210: protons plus neutrons, so neutrons \(= 210 - 84 = 126\). Its neighbour \({}^{209}_{84}\mathrm{Po}\) has one neutron fewer: same element, different isotope, very different half-life. Exam questions run this in both directions — notation to particle counts, particle counts to notation — and the mark scheme wants the subtraction A − Z written down, not just the answer.
CaseHow the model was won — Dalton to Chadwick
Every atom diagram you have ever drawn is a hypothesis that survived. John Dalton's atoms, in the early 1800s, were indivisible solid spheres. J.J. Thomson broke them in 1897 by discovering the electron — a particle far lighter than the lightest atom — and proposed the plum pudding model: a ball of positive charge with negative electrons embedded through it. In 1909 Hans Geiger and Ernest Marsden, working under Rutherford in Manchester, fired alpha particles at gold foil a few hundred atoms thick. Most passed straight through: the atom is mostly empty space. A few deflected sharply: the positive charge is concentrated, not spread out. And roughly 1 in 8,000 bounced back — Rutherford said it was as astonishing as an artillery shell rebounding off tissue paper — so nearly all the mass must sit in one tiny, dense, positive nucleus. By 1911 the plum pudding was dead.
Niels Bohr refined the wreckage in 1913: electrons cannot orbit just anywhere but only at fixed distances — energy levels — and his calculations matched experimental observations. Later work showed the nuclear positive charge comes in equal units (protons), and in 1932 James Chadwick proved the leftover nuclear mass belonged to a neutral particle, the neutron, about twenty years after the nucleus itself was accepted. The pattern is the exam point: each model stood only until new evidence broke it. And this exact story is common content with chemistry C1 — one revision, marks on two papers.
MechanismRadioactive decay and the four radiations
An unstable nucleus decays: it emits radiation and becomes more stable. The process is random — nothing can predict which nucleus will go next, or when — yet vast numbers of random events produce rock-steady statistics, which is why a source has a definite activity: the rate at which its nuclei decay, measured in becquerels (1 Bq = 1 decay per second). A Geiger–Müller tube intercepts only some of those decays; what it records is the count-rate, the clicks per second at the detector.
Four nuclear emissions are on the specification. An alpha particle is two protons and two neutrons — a helium nucleus: range about 5 cm in air, stopped by a sheet of paper, and the most strongly ionising, because it is heavy, doubly charged and relatively slow. A beta particle is a fast electron flung out of the nucleus as a neutron converts into a proton: around a metre of range in air, stopped by a few millimetres of aluminium, moderately ionising. A gamma ray is electromagnetic radiation from the nucleus itself: weakly ionising but hugely penetrating, tamed only by thick lead or metres of concrete. Some decays also eject neutrons. Notice the trade-off running one way through the list: the more ionising a radiation is, the shorter its reach. That is why the polonium in the hook was undetectable outside the body yet devastating inside it — and why the americium-241 in the smoke alarm on your ceiling, ionising a few millimetres of air inside its chamber, is harmless where it sits.
ModelNuclear equations — decay as double-entry bookkeeping
A decay is an equation, and it must balance twice over: the mass numbers on the right must sum to the mass number on the left, and the atomic numbers likewise. Alpha decay carries off two protons and two neutrons, so A falls by 4 and Z falls by 2 — the atom slides two places back in the periodic table and becomes a different element. Beta decay converts a neutron into a proton: A is unchanged, Z rises by 1 — one place forward. Gamma emission changes neither number; the nucleus merely sheds surplus energy. Because Z fixes the element, the periodic table in your exam paper names every decay product for you.
The hook's own chemistry, balanced. Polonium-210 is an alpha emitter: \[{}^{210}_{84}\mathrm{Po} \rightarrow {}^{206}_{82}\mathrm{Pb} + {}^{4}_{2}\mathrm{He}\] Top row: 206 + 4 = 210. Bottom row: 82 + 2 = 84. Both balance, and Z = 82 is lead: the poison decays, atom by atom, into stable lead-206. Beta decay runs the other way. Carbon-14, the dating isotope, decays as \[{}^{14}_{6}\mathrm{C} \rightarrow {}^{14}_{7}\mathrm{N} + {}^{0}_{-1}\mathrm{e}\] Mass number unchanged at 14; atomic number up one, from 6 to 7, because a neutron became a proton — and the electron's −1 keeps the bottom row honest: 7 + (−1) = 6. Run both checks on every nuclear equation you write; the marks are mechanical once you do.
DataHalf-life — random alone, exact in bulk
You cannot say when one nucleus will decay. You can say, with precision, how long half of any large sample will take: that is the half-life — the time for the number of unstable nuclei in a sample to halve, or equivalently for the activity or count-rate to fall to half its initial value. It is a fixed fingerprint of each isotope, spanning seconds to billions of years; polonium-210's is 138 days, and nothing — temperature, pressure, chemistry — changes it. The coin-toss analogy earns marks: any single toss is unpredictable, but half of a million coins reliably come up heads.
From a decay graph, extract the half-life with construction lines: take the initial count-rate, halve it, read across to the curve and down to the time axis. Then repeat from a different starting point — getting the same answer twice is the check that your curve really is exponential decay and your value is reliable.
Higher tier asks for the net decline as a ratio. A school source has activity 6,000 Bq and a half-life of 138 days. After one half-life: 3,000 Bq. After two: 1,500 Bq. After three — that is 414 days — 750 Bq. The fraction still undecayed is \(\left(\tfrac{1}{2}\right)^3 = \tfrac{1}{8}\), so the net decline ratio of initial to final activity is 6,000 : 750 = 8 : 1. Count the half-lives first, then apply the power of one-half. Never try to decay the number 'per day' — the fall is exponential, not linear, and linear reasoning is exactly the error the question is built to catch.
MechanismContamination, irradiation — and why peer review decides
Irradiation is exposure: radiation from an external source passes through you. It stops the moment the source is shielded or removed, and — the point examiners hammer — it does not make you radioactive, any more than light makes you glow after the lamp goes off. Hospitals rely on this: surgical instruments and some foods are sterilised with intense gamma doses and are perfectly safe to handle afterwards. Contamination is possession: unwanted radioactive atoms themselves, on skin, on clothes, or inside the body. A contaminated object keeps emitting until those atoms decay or are physically removed — and they transfer by contact, which is exactly how detectives could follow the polonium from teapot to hotel room to aircraft seat across London.
Which hazard is worse depends on the radiation type. A gamma source is the greater irradiation threat, because it reaches you across a room; an alpha source is nearly harmless a metre away and the worst possible contaminant once inside the body. Hence the precautions, each matched to a hazard: sealed sources handled with tongs at arm's length; gloves and aprons against contamination; exposure times kept short; and film badges recording the dose that radiographers and nuclear workers accumulate. One final spec point, easily forgotten and easily banked: findings on radiation risk are trusted because they are published and peer reviewed — checked by other scientists — before anyone builds safety rules on them.
VocabularyKey terms the mark scheme pays for
TrapsMisconceptions that cost marks
ExamWhat examiners want
Nuclear equations are the most mechanical marks in P4: write the equation, then run the two balance checks aloud — mass numbers sum across the arrow, atomic numbers sum across the arrow — and use the periodic table printed in the paper to name the product from its new atomic number. Alpha means A − 4 and Z − 2; beta means A unchanged and Z + 1 because a neutron became a proton; state that conversion explicitly, since it carries its own mark. On half-life graphs, draw your construction lines on the graph — halve the initial count-rate, across, then down — and read a second pair to show the value is consistent; Higher-tier ratio questions want half-lives counted first, then \(\left(\tfrac{1}{2}\right)^n\) applied, with the net decline expressed as a ratio like 8 : 1.
The alpha-scattering six-marker is a mapping exercise, and level-marked answers must link each observation to its inference rather than list facts: most alpha particles passed straight through → the atom is mostly empty space; some deflected → the positive charge is concentrated; a tiny fraction rebounded → nearly all the mass sits in a tiny, dense, positive nucleus. Three linked pairs, in order, plus the conclusion that the plum pudding model had to go. P4 is unusually recall-heavy — the AO balance across the paper is roughly 40% recall, 40% application, 20% analysis, and this topic supplies much of the first 40 — so learn the definitions of activity, half-life, isotope, contamination and irradiation word-perfectly; 'time for half the sample to decay' without 'unstable nuclei' or 'count-rate' is how one-mark definitions become zero. On compare questions, define both terms, state the hazard difference, and give one matched precaution each. Describe randomness with the approved sentence: you cannot predict which nucleus will decay or when, but large samples decay at a predictable rate. And a boundary warning worth revision hours: background radiation, radiation dose in sieverts, nuclear fission and fusion, and the medical uses of radiation all belong to separate physics — Trilogy's P4 ends at contamination, so spend the saved time on nuclear equations and half-life graphs instead.